Assuming all of the basic exercises have been taught correctly and the student is up to standard, the only new thing that you are trying to learn in the circuit is the take off and landing. In most cases it is only the landing you are trying to perfect.
16 hours implies a more fundamental problem than just the landing and perhaps it is time to go back to some of the basic exercises to ensure that the basic skills are up to speed. If they are not, then progress will be slow.
Circuit flying is a critical phase and having more concentrated lessons would be agood idea to try get past this hurdle.
I have flown with numerous students in a similar position and usually find that watching one circuit tells all. In most cases the problem is basic flying skills that need to be or improved, or have not been taught properly. This is best done away from the circuit.
Try flying with another instructor.